1. Field of the Invention
The present invention relates to a fuel supply apparatus for supplying fuel to an internal combustion engine.
2. Description of Related Art
JP2003-176761A, which corresponds to EP1319821A2, discloses a fuel supply apparatus. The fuel supply apparatus is located between a fuel tank and a diesel engine (hereafter referred to just as an engine). The fuel supply apparatus supplies fuel from the fuel tank through a fuel filter to constituents of a high-pressure fuel system, which include a common rail and fuel injection valves. The fuel supply apparatus returns excessive fuel (hereafter referred to as return fuel) from the constituents of the high-pressure fuel system to the fuel tank. This kind of fuel supply apparatus has a low-pressure fuel feed pump (hereafter referred to as a feed pump) and a high-pressure fuel supply pump. The feed pump draws up the fuel from the fuel tank and preliminarily pressurizes the fuel. The high-pressure fuel supply pump further pressurizes the fuel preliminarily pressurized by the feed pump. The above-mentioned return fuel is relatively hot. Therefore, the return fuel is returned to the fuel tank and is cooled by the fuel in the fuel tank.
As a type of the above-mentioned fuel supply apparatus, the fuel supply apparatus disclosed in JP2003-176761A has an introducing passage in a fuel circuit, which has a fuel filter on an upstream side of the feed pump. The introducing passage introduces the return fuel from the constituents of the high-pressure fuel system to an inlet side of the fuel filter. In this fuel supply apparatus, the fuel filter is located on the upstream side of the feed pump. Therefore, it is possible to introduce the relatively hot return fuel to the fuel filter with reliability, by using a pressure difference between pressure of the fuel at the inlet side of the fuel filter, which is generally a negative pressure, and pressure of the return fuel, which is approximately equal to atmospheric pressure.
JP57-156068U discloses another kind of fuel supply apparatus. This fuel supply apparatus has an electric heater on an inlet side of the fuel filter, that is, in a fuel passage between the fuel filter and the fuel tank.
EP0819844A2 discloses still another kind of fuel supply apparatus. This fuel supply apparatus has an introducing passage in a fuel circuit, which has a fuel filter on a downstream side of the feed pump. The introducing passage introduces the return fuel to an inlet side of the fuel filter. In this fuel supply apparatus, pressure of the fuel at the inlet side of the fuel filter is generally a positive pressure. This fuel supply apparatus aims to introduce the return fuel to the inlet side of the fuel filter where the pressure of the fuel is a positive pressure.
The inventors of the present invention have studied a possibility of locating the fuel filter on the downstream side of the feed pump in order to raise the pressure of the fuel that acts on the fuel filter. If an engine is started in a relatively low-temperature condition, viscosity of the fuel is high, and wax can precipitate out of the fuel. Therefore, clogging of precipitated wax (hereafter referred to just as wax clogging) can occur in the fuel filter. If the fuel filter is located on the downstream side of the feed pump, the fuel is pressure-fed through the fuel filter by the raised fuel pressure, and it is possible to delay occurrence of the wax clogging. However, there is an apprehension that the wax clogging can occur sooner or later.
In this regard, the fuel supply apparatus disclosed in JP2003-176761A may be applied to this kind of fuel circuit, which has the fuel filter on a downstream side of the feed pump. However, the pressure of the fuel at the inlet side of the fuel filter does not become lower than the pressure of the return fuel. Accordingly, the fuel supply apparatus of JP2003-176761A cannot be realized when it is applied the fuel circuit that has the fuel filter on the downstream side of the feed pump.
If the fuel supply apparatus disclosed in JP57-156068U is applied to the fuel circuit that has the fuel filter on the downstream side of the feed pump, it is possible to avoid the wax clogging in the fuel filter. However, this case requires an electric heater, and parts and electrical wirings become necessary to form a heat source on the inlet side of the fuel filter. Therefore, a cost of the fuel supply apparatus is considerably increased.
If the fuel supply apparatus disclosed in EP0819844A2 is applied to the fuel circuit that has the fuel filter on the downstream side of the feed pump, it is necessary to keep the pressure of the return fuel higher than the pressure of the fuel at the inlet side of the fuel filter. In this case, the pressure of the return fuel acts on a discharge side of the feed pump as a back pressure. Therefore, the pressure of the return fuel can lower discharge efficiency of the feed pump and can upsize the feed pump in some cases.